ECONOMIC GEOLOGY
11-inch diameter iron cylinder. The stones are tested both
dry and wet.
The tests for road metal recommended by the United States
Department of Agriculture and the Canadian Geological
Survey (Mem. G.S. Canada, 8s, 1916, by L. Reinecke and
114, 1919, by H. Gauthier) are based upon the specific gravity,
the percentage of wear under attrition ina rotating cylinder,
hardness measured by loss in weight when pressed against
a revolving iron disc armed with coarse sand, the absorption
of water by 48 hours’ immersion, and toughness. The grade
of toughness is tested by the height in centimetres through
which a two-kilogram hammer must fall to break a cylinder
of the stone, an inch in diameter and height.
According to the U.S. Office of Public Roads, stone for
roads that are traversed by less than 100 vehicles a day may
have a toughness of 5-0, by 100 to 250 vehicles a day of 10-13,
and by more than 250 vehicles a day a toughness of 10-19,
according to the different setting of the macadam.

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